Near-infrared absorptive dyes are used for various purposes in wide fields. The dyes are used in, for example, infrared-cutting films for plasma display panels (PDP) or CCDs, optical filters in heat ray shield films, or photothermal conversion materials in write once optical disks (CD-R) or flash-meltable and fixable materials. Moreover, the dyes are used as information displaying materials for security inks or invisible bar code inks. This security ink gives an image that is hardly colored by rays in the visible range and shows a large absorption for rays in the near infrared range. By use of this property, the security inks are utilized to print encrypted data (such as a bar code, a two-dimensional code, or OCR characters) detectable through near infrared rays (wavelength: 700 to 1800 nm) on prepaid cards, paper moneys, vouchers, valuable instruments (securities), plastic financial cards, bankbooks, insurance policies, any other certificates, or the like, in order to prevent forgery. Alternatively, the security inks are printed in the same manner, and used for reading with OCR on the basis of detection through near infrared rays, for location identification, for preventing malfunction of instruments, or for some other purpose. The inks are used also as hidden inks, which are inconspicuous and not to damage the design of a printed material, besides a forgery-preventing ink.
For near-infrared absorptive dyes, which are used for such information-display, it is very important that the dyes are excellent in invisibility, i.e., the property that the dyes are invisible, as well as the dyes show an intense absorption for rays in the near infrared range. Furthermore, for all dyes, a high durability (fastness) is required. Near-infrared absorptive dyes high in either of invisibility or high durability are disclosed (see JP-A-8-143853 (“JP-A” means unexamined published Japanese patent application), JP-A-10-60409, JP-A-7-164729, JP-A-11-279465, JP-A-2008-291072, JP-A-2002-146254, JP-A-2006-78888); however, near-infrared absorptive dyes made invisibility and durability compatible with each other are not developed. Thus, near-infrared absorptive dyes having both of these performances have been intensely desired.
As dyes which hardly show an absorption in the wavelength range of 400 to 700 nm and are excellent in invisibility, cyanine methine dyes or J associations thereof can be mentioned. However, their long methine conjugated chain is flexible; thus, the dyes are easily isomerized so that the absorption wavelengths are changed, or the dyes are easily decomposed by heat or by reaction with oxygen or a nucleophilic agent. Accordingly, the dyes are low in fastness.
As near-infrared absorptive dyes having a rigid skeleton and a high fastness, there are vanadylphthalocyanine dyes suggested by Nippon Shokubai Co., Ltd., and quaterrylene dyes suggested by BASF AG. However, vanadylphthalocyanine dyes are insufficient in invisibility. The quaterrylene dyes have good invisibility in a state of molecular dispersion, for example, in a state of a solution of the dyes; however, when the concentration is made high, an absorption is generated for rays in the visible range by the association of the molecules so that the invisibility is lost. Thus, the way of use thereof is restricted.
As dyes that are excellent in invisibility and have an absorption for rays in the infrared range widely, there are diimmonium dyes marketed by Nippon Kayaku Co., Ltd. However, the dyes are easily reducible and are insufficient in fastness. Thus, the way for use thereof is restricted.
As described above, near-infrared absorptive dyes having both excellent invisibility and fastness have not been marketed up to date. Thus, it has been desired to develop near-infrared absorptive dyes having these performances compatibly with each other.
In the meantime, JP-A-11-231126 and JP-A-2002-138203 describe, as an example of the application of near-infrared absorptive dyes, a film having a near-infrared absorptive filtering function and produced by dissolving or dispersing an organic pigment together with a binder resin into an organic solvent, and then applying the resultant solution or dispersion onto a film.
However, this mode of use has a drawback that the environment suffers adversely largely since organic solvents are used. Additionally, facilities for the production are required to be rendered explosion-preventing facilities. Thus, very large costs are required for facility investments.
Furthermore, as given as an example in JP-A-11-109126 and JP-A-2001-228324, the following is proposed: a water-soluble dye is dissolved together with a water-soluble binder, specifically gelatin, into water, and a film is coated with the resultant solution to produce a film having a near-infrared absorptive filtering function; and the resultant film is used in a PDP.
However, in connection with the water-soluble dyes coatable onto a film without using a large amount of organic solvents, there remains a problem that the resultant film coated with the dye is low in durability and the near-infrared absorptivity, in particular, at high temperature and high humidity, and other problems. Thus, the film product has been desired to be improved.
As a near-infrared absorptive ink, known is an organic-solvent-type ink, wherein a dissolvable near-infrared absorptive dye is dissolved in an organic solvent. However, the ink is low in durability. Thus, the ink has a problem that the ink decomposes in a short term so as not to show any absorption so that the ink comes not to be recognized. JP-A-7-164729 and JP-A-6-248213 propose a method of using an acrylic binder resin to improve the durability, or a method of adding an ultraviolet absorbent. However, the methods are insufficient. Additionally, the methods have a problem about safety or environmental hygiene since organic solvents are used. Thus, aqueous inks have been desired.
JP-A-2002-146254, JP-A-2002-187955, JP-A-9-263717, and JP-A-2002-309131 describe examples of aqueous near-infrared absorptive-paint and -ink. However, the examples each have problems about durability or invisibility, since naphthalocyanine, diimmonium, or croconium dyes are used.
It is proposed that an infrared absorbing ink containing an inorganic ion (such as a copper, iron, or ytterbium ion) be used to form an infrared absorbing image (see JP-A-8-143853 and JP-A-10-60409). However, the ink is sufficient in the infrared absorbency of the image but is insufficient in invisibility. When the invisibility is made high, the infrared absorbency conversely becomes insufficient. Thus, when the invisible image is intermingled with a visible image, the discriminatability of the ink becomes insufficient. Furthermore, the heavy metal contained in the ink imposes a heavy load on the environment.
Furthermore, known are examples wherein an infrared absorbing ink containing a phthalocyanine dye, a dithiol compound dye, a squarylium dye, a croconium dye, or a nickel complex dye is used, to form an infrared absorbing image (see JP-A-7-164729, JP-A-11-279465, JP-A-2008-291072, and JP-A-2002-146254). A toner containing an infrared absorbent composed of a diimmonium compound is also investigated (see JP-A-2006-78888). A toner containing a near infrared absorbent can be used as an invisible toner. The toner may be used to form an invisible image, such as a code pattern as described above. However, the inks or toners disclosed in JP-A-7-164729, JP-A-11-279465, JP-A-2008-291072, JP-A-2002-146254, and JP-A-2006-78888 are still insufficient in compatibility of invisibility with durability.